Background doesn't render transparently despite checking Film-->Transparent; RGBA settings (cycles, v2.83)
You are using an Alpha Over node and mixing something over the image messing up the alpha as well. Just plug the correct alpha you want to the output to fix this:

If you need glow, you should add it instead of mixing it. Using alpha will not give you the correct results in other application if you need glow. Glow is made of light and it should be added to the image, not mixed with it, so you should make a separate glow image to add it in whatever software you are using for compositing over the background and the object placed on the background.